Occurrence of antibiotic resistance among bacilli in Brazilian soils and the possible involvement of resistance plasmids

Summary

Numbers of bacilli resistant to each one of 6 antibiotics were determined in samples of 5 different Brazilian soils. In all soils, antibiotic-resistant cells made up only a minor part of the total population of bacilli. Resistance to penicillin (Pc), streptomycin (Sm) and kanamycin (Km) was most frequent in the majority of soils, however, numbers of bacilli resistant to erythromycin (Em) and tetracyclin (Tc) were substantially lower. Chloramphenicol (Cm) resistance showed intermediate levels.

Thirty-four strains resistant to either Tc, Em or Cm were isolated and identified to species level. A screening for the presence of extrachromosomal DNA in these strains revealed that 11 carried one or more plasmids in the molecular weight range 2.3–30 Md. A 2.8 Md plasmid, pFT-30, detected inB. cereus, was shown to carry a Tc resistance determinant. The antibiotic resistance of non of 8 other plasmid-containing strains could be linked, with certainty, to the presence of the plasmid. However, preliminary evidence pointed to the localization of Em resistance determinants in 2B. subtilis strains on plasmids of 3.2–3.4 Md.